linux-kernel - Re: Wrong DIF guard tag on ext2 write

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Date:	Tue, 01 Jun 2010 16:54:53 +0000
From:	James Bottomley <James.Bottomley@...e.de>
To:	Chris Mason <chris.mason@...cle.com>
Cc:	Matthew Wilcox <matthew@....cx>,
	Christof Schmitt <christof.schmitt@...ibm.com>,
	Boaz Harrosh <bharrosh@...asas.com>,
	"Martin K. Petersen" <martin.petersen@...cle.com>,
	linux-scsi@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-fsdevel@...r.kernel.org
Subject: Re: Wrong DIF guard tag on ext2 write

On Tue, 2010-06-01 at 12:47 -0400, Chris Mason wrote:
> On Tue, Jun 01, 2010 at 10:29:30AM -0600, Matthew Wilcox wrote:
> > On Tue, Jun 01, 2010 at 09:49:51AM -0400, Chris Mason wrote:
> > > > I agree that a block based retry would close all the holes ... it just
> > > > doesn't look elegant to me that the fs will already be repeating the I/O
> > > > if it changed the page and so will block.
> > > 
> > > We might not ever repeat the IO.  We might change the page, write it,
> > > change it again, truncate the file and toss the page completely.
> > 
> > Why does it matter that it was never written in that case?
> 
> It matters is the storage layer is going to wait around for the block to
> be written again with a correct crc.

Actually, I wasn't advocating that.  I think block should return a guard
mismatch error.  I think somewhere in filesystem writeout is the place
to decide whether the error was self induced or systematic.  For self
induced errors (as long as we can detect them) I think we can just
forget about it ... if the changed page is important, the I/O request
gets repeated (modulo the problem of too great a frequency of changes
leading to us never successfully writing it) or it gets dropped because
the file was truncated or the data deleted for some other reason.

James


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